1. Field of the Invention
The invention relates to an electrophotographic photoreceptor (also referred to below simply as a “photoreceptor”), a method of producing such a photoreceptor, an electrophotographic device, and a method of producing a copolymeric polyarylate resin. More specifically, the invention relates to an electrophotographic photoreceptor which is composed primarily of an electrically conductive substrate and a photosensitive layer containing an organic material, and which is adapted for use in electrophotographic printers, photocopiers, fax machines and the like, to a method of producing such a photoreceptor, to an electrophotographic device, and to a method for producing a copolymeric polyarylate resin.
2. Background of the Related Art
Electrophotographic photoreceptors have a basic structure in which a photosensitive layer with a photoconductive function is provided on an electrically conductive substrate. In recent years, organic electrophotographic photoreceptors using organic compounds as functional components for generating and transporting electrical charges have been the subject of active research and development owing to their diversity of materials, high productivity, safety and other advantages, and they are being applied to copiers, printers and the like.
In general, a photoreceptor must have the function of holding a surface charge in a dark place, the function of receiving light and generating charge, and also the function of transporting the generated charge. Such photoreceptors include single-layer photoreceptors provided with a single photosensitive layer endowed with all of these functions, and also stacked (functionally separated) photoreceptors provided with a photosensitive layer comprising a stack of functionally discrete layers: primarily, a charge generating layer that serves the function of generating charge during photoreception, and a charge transport layer that serves the function of holding a surface charge in a dark place and transporting the charge generated in the charge generating layer during photoreception.
The photosensitive layer is normally formed by dissolving or dispersing a charge generating material, a charge transporting material and a resin binder in an organic solvent to obtain a coating liquid that is then applied onto a conductive substrate. In these organic electrophotographic photoreceptors, polycarbonates that are highly flexible, transparent to light exposure and resistant to friction with the paper and the blade used for toner removal are often used as resin binders in the layer forming the outermost surface in particular. Of these, bisphenol Z polycarbonate is widely used as a resin binder. Art using this polycarbonate as a resin binder is described in, for example, Japanese Patent Application Laid-open No. S61-62040, Patent Document 1.
Currently, most electrophotographic devices are so-called digital devices which use monochromatic light from, for example, an argon, helium-neon or semiconductor laser or a light-emitting diode as an exposing light source to digitize and convert images, lettering and other information to optical signals. These optical signals are irradiated onto an electrically charged photoreceptor so as to form on the photoreceptor surface an electrostatic latent image that is then rendered visible with a toner.
Methods of charging the photoreceptor include non-contact charging systems which use charging devices such as scorotrons that do not contact the photoreceptor, and contact charging systems which use charging devices with semiconductive rubber rollers or brushes that contact the photoreceptor. The advantage of a contact charging system over a non-contact charging system is that, because the corona discharge occurs very near the photoreceptor, little ozone is generated and little applied voltage is required. Hence, this system is favored in medium-sized and small devices in particular because it enables an electrophotographic device that is compact, inexpensive and environmentally friendly to be obtained.
The most common methods for cleaning the surface of the photoreceptor include scraping with a blade and a simultaneous cleaning/development process. In the case of blade cleaning, untransferred residual toner on the surface of the organic photoreceptor is scraped off with a blade, and the toner is then collected in a waste toner box or returned to the developing unit. The difficulty with cleaning by this blade scraping system is that space is required for the toner collection box and recycling, and it is necessary to monitor the amount of toner in the toner collection box. Moreover, if paper dust and external additives accumulate on the blade, they may damage the surface of the organic photoreceptor, shortening the life of the electrophotographic photoreceptor. Hence, the toner is sometimes collected in the development process, or a means for magnetically or electrically drawing off residual toner adhering to the surface of the electrophotographic photoreceptor is installed immediately before the developing roller.
When using a cleaning blade, the rubber hardness must be enhanced and the contact pressure must be increased in order to improve the cleaning properties. However, this promotes wear of the photoreceptor, causing fluctuations in potential and sensitivity, which leads to image abnormalities and, in the case of color devices, problems with color balance and reproducibility.
In the case of a cleaningless system in which development and cleaning are performed together by a development unit using a contact charging mechanism, toner with a fluctuating charge quantity is produced in the contact charging mechanism. Another problem is that when a very small amount of reverse-polarity toner is present in the toner, these toners cannot be sufficiently removed from the photoreceptor and contaminate the charging device.
The surface of the photoreceptor surface may also be contaminated by ozone, nitrogen oxides and the like produced during charging of the photoreceptor. In addition to image deletion caused by the contaminants themselves, adhering substances may reduce the lubricity of the surface, making it easier for paper dust and toner to adhere to the surface and causing other problems such as blade noise, curling and surface scratches.
To increase toner transfer efficiency in the transfer step, attempts have been made to improve transfer efficiency and thus reduce the amount of residual toner by optimizing the transfer current for the properties of the paper and the temperature and humidity environment. Organic photoreceptors with improved toner release properties and organic photoreceptors with a smaller transfer effect are thus needed as organic photoreceptors suited to such processes and contact charging systems.
To resolve these problems, various methods have been proposed for improving the surfacemost layer of the photoreceptor. For example, Japanese Patent Application Laid-open Nos. H1-205171 and H7-333881, respectively, Patent Documents 2 and 3, propose methods for adding a filler to the surface layer of the photosensitive layer in order to increase the durability of the photoreceptor surface. However, with these methods of dispersing a filler in the film, it is difficult to disperse the filler uniformly. Filler aggregates also occur, film transparency is reduced and the filler scatters exposure light, causing irregularities in charge transport and charge generation and detracting from the image characteristics. One method of improving filler dispersibility is to add a dispersant, but in this case the dispersant itself affects the photoreceptor characteristics, which are thus difficult to reconcile with filler dispersibility.
Japanese Patent Application Laid-open No. H4-368953, Patent Document 4, discloses a method for including a fluoroplastic such as polytetrafluoroethylene (PTFE) in the photosensitive layer, and Japanese Patent Application Laid-open No. 2002-162759, Patent Document 5, discloses a method for adding a silicone resin such as an alkyl-modified polysiloxane. However, in the method of Patent Document 4, the PTFE or other fluoroplastic has a low solubility in solvents or a poor compatibility with other resins, causing phase separation and light scattering at resin interfaces. As a result, the sensitivity characteristics have fallen short of what is desired in a photoreceptor. With the method of Patent Document 5, the problem has been that continuous effects are not obtained because the silicone resin bleeds to the surface of the coating film.
To solve these problems, Japanese Patent Application Laid-open No. 2002-128883, Patent Document 6, proposes a method for improving wear resistance by using a resin in which a siloxane structure has been added to the terminal structure. Japanese Patent Application Laid-open No. 2007-199659, Patent Document 7, describes a photoreceptor containing a polycarbonate or polyarylate obtained using a specific siloxane structure-containing phenol as a starting material. Japanese Patent Application Laid-open No. 2002-333730, Patent Document 8, discloses a photoreceptor containing a resin having a carboxyl group-containing siloxane resin structure introduced into the resin structure. Japanese Patent Application Laid-open No. H5-113670, Patent Document 9, describes a photosensitive layer containing a polycarbonate the surface energy of which has been reduced by the inclusion of a silicone structure. Japanese Patent Application Laid-open No. H8-234468, Patent Document 10, describes a photoreceptor which contains a polyester resin that includes polysiloxane as structural units on the surfacemost layer of the photoreceptor.
Japanese Patent Application Laid-open No. 2005-115091, Patent Document 11, discloses the use of polyarylate as a resin binder for the photosensitive layer, and describes repeated investigations conducted for such purposes as to improve durability and mechanical strength. Japanese Patent Application Laid-open No. 2002-214807, Patent Document 12, proposes a photoreceptor wherein polycarbonate and polyarylate resins having siloxane structures are used in the photosensitive layer by using a phenol-modified polysiloxane resin as the siloxane component. Japanese Patent Application Laid-open No. 2004-93865, Patent Document 13, discloses an electrophotographic device having a silicone-modified polyarylate resin-containing photosensitive layer. Japanese Patent Application Laid-open Nos. 2007-121751 and 2010-96929, respectively, Patent Documents 14 and 15 disclose photoreceptors which use polyarylate resins in the photosensitive layer.
Methods have also been proposed for forming surface protective layers on the photosensitive layer with the aim of protecting the photosensitive layer and improving mechanical strength and surface lubricity. However, the problem with these methods of forming a surface protective layer has been the difficulty of forming a film on the charge transport layer and the difficulty of satisfactorily achieving both charge transporting properties and a charge holding function.
Unfortunately, it is not possible with the art disclosed in the above-cited Patent Documents to achieve both a low coefficient of friction and a low amount of wear. Moreover, this foregoing art is also inadequate for maintaining good electrical characteristics and image characteristics.
It is therefore an object of the invention to provide an electrophotographic photoreceptor which, by enabling the amount of wear on the surface of the photoreceptor drum to be reduced and also lowering the coefficient of friction from the beginning until after printing, is able to obtain good images. Further objects of the invention are to provide a method of producing such a photoreceptor, an electrophotographic device, and a method of preparing a copolymeric polyarylate resin.